1. Field of the Invention
The present invention relates to transconductance amplifiers, and more particularly to high current transconductance amplifiers that are stable over a very wide bandwidth.
2. Related Prior Art
Transconductance amplifiers are widely used as a programmable source of current to calibrate meters, transformers, and shunts, in addition to experimental uses requiring a stable source of constant current. Such an amplifier ideally produces a current in a load proportional to an input voltage and maintains that current independent of the load impedance. There is an increasing need to provide calibrations of current transformers, meters, and shunts at current levels up to and even exceeding twenty amps rms at 100 kHz. Although commercial transconductance amplifiers exist which can deliver currents well in excess of twenty amps, they fail to provide such levels of current above just a few kHz. One of the main limitations to the conventional transconductance design approach is the current-sensing resistor, which for practical reasons, must be in the milli-ohm range for currents in the tens of amperes. Low-value shunt resistors of this order must be of special design to minimize reactance, deviation of resistance with frequency, and temperature effects of resistance.
Prior art has described a variety of fixed and controlled current sources implemented by combinations of monolithic and discrete components. Almost all transconductance amplifier designs or current sources rely on some type of current shunt to provide a voltage proportional to current which is used as feedback to compare it with an input voltage. The shunt must be properly designed to provide a voltage drop across it that is a true measure of the current through it over a wide frequency range with minimum phase shift. At low currents, rather ordinary commercial metal film resistors serve well. However, practical considerations dictate a low-value resistance shunt for high currents. Typically, a resistance of about five to ten milli-ohms is used for current levels in the twenty amp range. Low-value shunt resistors of this order must be of special design to minimize their reactance, deviation of resistance with frequency, and temperature coefficient of resistance. Although good low-value shunt resistors are achievable, they are often proprietary items and expensive because of the special design necessary.